The present invention relates to an electronic sphygmomanometer or to an automatic indirect blood pressure measuring device.
An automatic indirect blood pressure measuring device has been developed, which utilizes the Korotkoff sounds, or blood vessel sounds derived from a microphone positioned under an arm cuff. In such a device, first a high pressure is applied to the arm cuff through the use of an air pump and, then, the pressure applied to the arm cuff is gradually reduced at a rate, for example, 2 to 4 mmHg/sec. During the reduction procedure of the applied pressure, the Korotkoff sounds appear at the systolic pressure point, and the Korotkoff sounds disappear at the diastolic pressure point.
A typical control system for an electronic sphygmomanometer is disclosed in U.S. Pat. No. 4,273,136, "ELECTRONIC SPHYGMOMANOMETER" issued on June 16, 1981.
In such an electronic sphygmomanometer, it is strictly required that the Korotkoff sounds are distinguished from noises in order to ensure an accurate detection of the blood pressure. In the conventional system, the microphone output signal is passed through a low-pass filter or a bandpass filter, and the filter output signal is applied to a comparator to determine the peak value of the obtained signal. However, the frequency characteristics of the Korotkoff sounds are variable depending upon the person to be measured. Especially, if a person has hardened arteries or thick subcutaneous fat, the determination as to whether the microphone output signal is the Korotkoff sounds or not is very difficult.
Accordingly, an object of the present invention is to provide a detection system for an electronic sphygmomanometer, which ensures an accurate detection of the blood pressure.
Another object of the present invention is to provide a digitally controlled detection system for an electronic sphygmomanometer, which ensures an accurate detection of the Korotkoff sounds.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. It should be understood, however, that the detailed description and specific examples, which indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
To achieve the above objects, pursuant to an embodiment of the present invention, the microphone output is applied to an A/D converter via a filter. The thus obtained digital signal is introduced into a microcomputer to check the characteristics of the digital signal. More specifically, the digital signal is applied to a determination circuit at a predetermined interval. The determination circuit first detects the leading edge peak and the trailing edge peak of the output signal derived from the A/D converter. Then, the determination is conducted as to whether the value difference between the leading edge peak and the trailing edge peak is greater than a preselected value, and as to whether the time interval between the peaks is within a preselected period of time.